1. Field of the Invention
The present invention relates to an improvement of a welding gun.
2. Description of the Prior Art
Japanese Patent Laid-Open Publication No. 62-64480 discloses a prior welding gun which will be described hereafter with reference to FIG. 3.
The prior welding gun comprises a fixed arm having an electrode at one end thereof, a movable arm 15 composed of an upper portion having an electrode at one end thereof which is opposed to the electrode of the fixed arm, and a lower portion fixed to the upper portion at one end thereof, and connected at the other end thereof to the distal end of the rod 14 of a piston in a cylinder by a link 22. The lower portion of the movable arm 15 has rollers 16, 17. A guide plate provided with the guide groove 18 for guiding guide rollers 16, 17 is fixedly mounted over the cylinder.
The guide groove 18 comprises a rod guide member 19 an axis of which is aligned with that of the rod 14, a middle guide member 20 an axis of which is spaced with a predetermined interval from and in parallel with that of the rod 14, and a circular arc member 21 disposed opposite to the rod guide member 19 and spaced with a predetermined interval.
The prior welding gun has an advantage that the movable arm can be moved away from the fixed arm with a large angle or a long distance by a slight reciprocal movement of the rod, but has a disadvantage that not only complicated components such as the guide roller, the guide plate, and the guide groove are required which entails in requiring a large-sized welding gun, but also many parts are required since a force applied to the piston in the cylinder during the backward movement of the rod is less than that during the forward movement of the rod for the cross sectional area of the rod. As a result, there is a likelihood that the pressing force will be reduced. To overcome the disadvantage, the cylinder will be large sized or the pressure of the operating fluid is increased.